Emma Thompson has taken on some difficult roles during her career, winning Oscars and numerous other awards for her performances in works based on classic literature and historical events. But, says the actress, one role stands apart from the rest.

“She’s the most difficult person I’ve ever played,” Thompson says of her role as P.L. Travers, the author of the Mary Poppins books,in the forthcoming film Saving Mr. Banks.

The movie, coming out this holiday season, is Disney’s take on Travers’ life and, eventually, the stormy relationship she had with the studio’s own mastermind, Walt Disney. Here, TIME presents an exclusive first official look at Emma Thompson as P.L. Travers and Tom Hanks as Walt Disney. In the scene shown here, Travers has come to Disneyland, in 1961, to meet with the man himself.

Of course, today’s audiences have long enjoyed their resulting cinematic collaboration: Disney’s 1964 Mary Poppins movie. But the film was a struggle to make: even though Disney had a personal interest in Travers’ tale of a magical nanny, it being one of his daughters’ favorites, the author was not interested in her work being changed by a movie studio.

“She was a woman of quite eye-watering complexity and contradiction,” says Thompson, whose first-ever movie-going experience was seeing a Disney movie, Fantasia, as a child. “Often I play people who are controlled by some very clear guiding moral principles. Like Margaret Schlegel [in Howards End], guided by the early principles of feminism and equal rights, and Elinor Dashwood [in Sense and Sensibility], guided by the principles of decency and honor. There are very clear moral prisms these women pour life through, and I understand that very well. And [Travers] was not like that at all. She was far more chaotic and confused and morally various.”

Travers, who died in 1996, wrote five Mary Poppins stories and many other books. She was a private person whose personal life has remained unknown to many who love her work. But Thompson, who is a writer as well as an actor (and wrote the screenplay for 2005’s Nanny McPhee), says that any Poppins fan knows one thing about Travers: her complete devotion to her books, characters and audience. Her emotional complexities notwithstanding, Travers’ wariness about the Mary Poppins movie was understandable.

“There’s a tendency to cast aside a children’s author or children’s film and make it seem less important than those for an adult audience. Of course, I think it’s the other way around,” says Thompson. “You can get away with dropping the ball from time to time with adult audiences but children know. You’ve really got to do your best. Letting them down is a serious failure in your artistic integrity.”